1. Field of the Invention
The present invention relates to a sheet for forming a sleeve to be used as a protective cover for a fragile article, such as a glass bottle.
2. Description of the Prior Art
Fragile articles, such as glass bottles, have been protected by covering the outer surface thereof with a sleeve comprising a shrinkable synthetic resin; applying heat to heatshrink the sheet so as to form a covering tightly connected to the outer surface of the article.
As shrinkable sheets for forming such a sleeve, foamed polystyrene resins have been used (see, for example, U.S. Pat. No. 3,760,968). With such polystyrene sheets, sleeves with good shock-absorbing properties can be obtained. However, such sleeves have a poor surface smoothness and printability and the surface thereof is susceptible to scratches. Further, glass bottles covered with a sleeve of heat-shrunk polystyrene resins have poor surface slippage properties so that, when continuously transported by a vibration conveyor or the like, they may not move smoothly and, in fact, may come to a halt.
Accordingly, it is an object of the present invention to provide a sheet for forming a sleeve which is not only free from such disadvantages, but also have excellent printability and surface smoothness.
On the other hand, in cases where heat-shrinkable sheets are applied to glass bottles for filling pressurized liquids, such as a carbonated beverage, it is important from the standpoint of safety to prevent the scattering of glass fragments upon the bursting thereof.
Various known protective sleeves of synthetic resin sheets have been used and subjected to a bottle bursting test, using the Japanese Industrial Standard S-2306 (Method of Fragment Retention Test for Carbonated Beverage Bottles) as a criterion of safety. As a result, it has been found that the bursting safety of such bottles (i.e., scattering of fragments) has an interrelationship with the tensile strength and elongation of the sheets employed as a protective cover.
More specifically, sleeve-forming sheets which satisfactorily pass the above bursting test are limited to only those having a tensile strength and elongation within a certain range, as shown in FIG. 5. FIG. 5 illustrates the results of the above bursting test, in which the axes indicate tensile strength and elongation of the sheets (measured in the peripheral direction thereof). It has also been found that sleeves made of sheets having a tensile strength or elongation smaller than a certain value fail to satisfactorily pass the test without exception. Also it has been found that sleeves made of sheets having a tensile strength or elongation exceeding a certain value may be quite dangerous, because when a bottle covered with such a sleeve bursts, fragments are scattered like shells in the axial direction of the bottle.
In the above-described tests, 300 ml glass bottles for carbonated beverages (weight: 170 g) were used. The tests were carried out in accordance with Japanese Industrial Standard S-2306, wherein the bottles were charged with an inner pressure of 4.0 to 4.5 Kg/cm.sup.2 and dropped by a height of 75 cm at a temperature of 25.degree. C. The tensile strength and elongation were measured on samples having a width of 10 mm and a length of 40 mm cut from the sleeves in the peripheral direction of the bottles.
Of the variety of synthetic resin sheets currently in practical use for shrinkable sleeves, polyvinyl chloride sheets, stretched polystyrene sheets, etc. can satisfy the requirements necessary to stand the above-described test.
In addition to the above-described bursting safety, protective sleeves are required to have good durability against vibrations and shocks which result, for example, during transportation of the bottles. However, the previously known sheets, including polyvinyl chloride and polystyrene sheets, are inferior in their ability to protect glass bottles against external forces and/or have inferior shock-absorbing properties. Furthermore, polyvinyl choride and stretched polyethylene sheets are also inferior in shrink-covering characteristics and the overall strengths thereof during use.
On the contrary, the foamed polystyrene sheets disclosed in U.S. Pat. No. 3,760,968 have excellent shock-absorbing properties, but have insufficient tensile strengths and elongations. The sheets, therefore, are inferior in safety upon bursting and lack practicability.
Accordingly, it is another object of the present invention to provide a sheet for forming a sleeve which has both superior shock-absorbing properties and bursting safety, as well as having good shrink-covering properties.